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We develop and thoroughly test a stress-controlled, parallel plates shear cell that can be coupled to an optical microscope or a small angle light scattering setup, for simultaneous investigation of the rheological properties and the microscopic structure of soft materials under an imposed shear stress. In order to minimize friction, the cell is based on an air bearing linear stage, the stress is applied through a contactless magnetic actuator, and the strain is measured through optical sensors. We discuss the contributions of inertia and of the small residual friction to the measured signal and demonstrate the performance of our device in both oscillating and step stress experiments on a variety of viscoelastic materials.

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We couple a laser-based, space-resolved dynamic light scattering apparatus to a universal testing machine for mechanical extensional tests. We perform simultaneous optical and mechanical measurements on polyether ether ketone, a semi-crystalline polymer widely used in the industry. Due to the high turbidity of the sample, light is multiply scattered by the sample and the diffusing wave spectroscopy (DWS) formalism is used to interpret the data. Space-resolved DWS yields spatial maps of the sample strain and of the microscopic dynamics. An excellent agreement is found between the strain maps thus obtained and those measured by a conventional stereo-correlation bench. The microscopic dynamics reveals both affine motion and plastic rearrangements. Thanks to the extreme sensitivity of DWS to displacements as small as 1 nm, plastic activity and its spatial localization can be detected at an early stage of the sample deformation, making the technique presented here a valuable complement to existing material characterization methods.

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The Wheat Grain as a cemented granular material: Nanoscale investigation of its mechanical properties. 3. International Conference on Structural Nano Composites (NANOSTRUC 2016)

Résumé:

We demonstrate the instability of the free surface of a soft elastic layer facing downwards. Experiments are carried out using a system consisting in a gel of constant density and shear modulus , put in a rigid cylindrical dish of depth h. When turned upside down, the free surface of the gel undergoes a normal outgoing acceleration g., whereas a steady pattern spontaneously appears in the opposite case. This phenomenon results from the interplay between the gravitational energy and the elastic energy of deformation of the system

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The confinement of colloidal particles in thin liquid films is typically accompanied with interfacial deformations which give rise to long-range capillary forces, attractive in most of the cases [1]. In simple fluids, these capillary forces compete mostly with electrostatic interactions. In thin complex fluid films such as a liquid crystal, other long-range interactions related to bulk distortions of the surrounding fluid appear. We have recently shown that the presence of a nematic order drastically changed the long-range interactions between particles [2] via an original elasto-capillarity coupling. In this work, we focus on the variety of distortions produced by the colloidal inclusions. In nematic films much thicker than the particles' size, a particle gives rise to elastic multipoles formed by the particle and nearby topological defects [3]. In thin nematic films with hybrid anchoring conditions at free surfaces, capillary effects strongly modify the behaviour of the elastic dipoles. For a certain range of film thickness values, two new interesting patterns emerged around isolated particles: the giant dipole [2] and the " butterfly " texture (see Fig.1). In the giant dipole, a micron-sized sphere is accompanied by a point defect located at a distance up to several hundreds microns. The situation is quite different in the " butterfly " texture: the particle still produces an accompanying defect in its close neighbourhood, but a pi-wall is formed on the opposite side. Using spatially resolved retardation and easy-axis maps, we analysed quantitatively and separately the 2D interfaces deformation and the nematic textures. Both behaviours are due to the same axisymmetric capillary deformation of the thin film around the beads but with different boundary conditions for the polar 2D c-director. Using numerical simulations, we were able to reproduce the patterns found in the films. Although capillary interactions between inclusions in a thin film are always attractive [4], these new spontaneous organizations in thin nematic films offer new ways to self-assemble complex colloidal systems in 2D. Figure 1: The two different birefringence patterns formed by microparticles trapped in a thin nematic film, observed a) between crossed polarizers and b) with an Abrio birefringence measurement system.[1] P.A. Kralchevsky, K. Nagayama K, Adv Colloid Interface Sci , 2000, 85, 145.[2] H.Jeridi, M.A.Gharbi, T.Othman, C.Blanc, Proc Natl Acad Sci USA, 2015, 112,14771.[3] I. Musevic, M.Skarabot, U.Tkaler, M.Ravnik and S.Zumer, Science, 2006, 313,954.

Résumé:

One of the major environmental issues related to spraying of pesticides on cultivated crops is the drift phenomenon. Because of the wind, small droplets may drift away from the targeted crop and cause contamination. One way to reduce the drift is to control the spray drop size distribution and reduce the proportion of small drops. In this context, anti-drift additives have been developed, including dilute oil-in-water emulsions. Although being documented, the effects of oil-in-water emulsions on spray drop size distribution are not yet understood. The objective of this work is to determine the mechanisms at the origin of the changes of the spray drop size distribution for emulsion-based sprays. Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle. At the exit of the nozzle, a free liquid sheet is formed, which is subsequently destabilized into droplets. In order to elucidate the mechanisms causing the changes of the spray drop size distribution, we investigate the influence of emulsions on the destabilization mechanisms of liquid sheets. Model single-tear experiments based on the collision of one tear of liquid on a small solid target are used to produce and visualize liquid sheets with a fast camera. Upon impact, the tear flattens into a sheet radially expanding in the air bounded by a thicker rim. Different destabilization mechanisms of the sheet are observed depending on the fluid properties. A pure water sheet spreads out radially and then retracts due to the effect of surface tension. Simultaneously, the rim corrugates forming radial ligaments, which are subsequently destabilized into droplets. The destabilization mechanism is drastically modified when a dilute oil-in-water emulsion is used. Emulsion-based liquid sheets are destabilized through the nucleation of holes within the sheet that perforate the sheet during its expansion. The holes grow until they merge together and form a web of ligaments, which are then destabilized into drops. The physical-chemical parameters of the emulsion, such as emulsion concentration and emulsion droplet size distribution, are modified to rationalize their influence on the perforation mechanism. We correlate the size distribution of drops issued from conventional agricultural spray with the amount of perforation events in single-tear experiments, demonstrating that the single-tear experiment is an appropriate model experiment to investigate the physical mechanisms governing the spray drop size distribution of anti-drift formulations. We show that the relevant mechanism causing the increase of drops size in the emulsion-based spray is a perforation mechanism. To gain an understanding of the physical mechanisms at the origin of the perforation events, we develop an optical technique that allows the determination of the time and space-resolved thickness of the sheet. We find that the formation of a hole in the sheet is systematically preceded by a localized thinning of the liquid film. We show that the thinning results from the entering and Marangoni-driven spreading of emulsion oil droplet at the air/water interface. The localized thinning of the liquid film ultimately leads to the rupture of the film. We propose the perforation mechanism as a sequence of two necessary steps: the emulsion oil droplets (i) enter the air/water interface, and (ii) spread at the interface. We show that the formulation of the emulsion is a critical parameter to control the perforation. The addition of salt or amphiphilic copolymers can trigger or completely inhibit the perforation mechanism. We show that the entering of oil droplets at the air/water interface is the limiting step of the mechanism. Thin-film forces such as electrostatic or steric repulsion forces stabilize the thin film formed between the interface and the approaching oil droplets preventing the entering of oil droplets at the interface and so inhibit the perforation process.